To enhance the motion stability of a vehicle, it has been proposed to individually control the traction and/or braking force of a right and left wheel of the vehicle, and thereby create a yaw moment for the vehicle. Such a system is known as the active torque transfer system (ATTS). When such a system is operated, the fore-and-aft forces acting upon the wheels may be varied from one wheel to another. The fore-and-aft force acting upon each front wheel creates a moment around the king pin thereof. Therefore, when the fore-and-aft forces acting upon the front wheels are unevenly distributed between the two wheels, the moments around the king pins are not balanced, and this causes a steering torque which will be referred to as torque steer hereinafter. This causes irregular changes in the steering torque, and may cause some discomfort to the vehicle operator. Therefore, it has been proposed to use an electric power steering system which is configured to produce an assist steering torque also for counteracting such a torque steer. See Japanese patent laid open publication No. 11-129927 (patent document 1), for instance.
An electric power steering system is typically incorporated with a failure detection system which prevents unpredictable steering assist torque from being produced by taking appropriate measures when a failure of the electric power steering system is detected. It can be accomplished by detecting a failure when the direction of the steering torque and the direction of the steering assist torque persist to differ from each other for more than a prescribed time period. When a failure is detected, the operation of the electric motor that provides the power assist may be prohibited. See Japanese patent laid open publication No. 11-059446 (patent document 2), for instance.
Japanese patent laid open publication No. 2001-130434 (patent document 3) discloses a failure detection system for an electric power steering system in which an assist torque demand from another system is added to the assist steering torque to determine the final target current for the electric motor for providing the power assist. A failure of the electric power steering system is detected when the steering assist torque and electric current fed to the electric motor continue to meet a certain failure detection condition for more than a prescribed first time period, and the activation of the electric motor is prohibited when a failure is detected. However, when the steering assist torque and electric current fed to the electric motor continue to meet a certain failure detection condition for more than a prescribed second time period which is shorter than the first time period, without prohibiting the operation of the electric motor, the assist torque demand from the other system is ignored. Thereby, the operation of the electric power steering system is maintained as long as possible before finally detecting a failure and shutting off the electric power steering system.
These prior inventions have some problems. In the case of the invention disclosed in patent document 2, when an active torque transfer system has created a yaw moment by suitably distributing traction to different wheels, and a torque steer that tends to reduce the steering torque is produced, the direction of the torque steer opposes that of the assist steering torque which is proportioned to the steering torque. Therefore, when the torque steer is large enough, the directions of the manual steering torque and assist steering torque could oppose each other, and this may cause the fault detection system of the electric power steering system to prohibit the operation of the electric motor for providing the power assist.
In the case of the prior invention disclosed in patent document 3, when the assist torque demand from the other system is abnormal, and a failure is detected as a result, the assist torque demand is ignored, and the assist steering torque may continue to be produced so as to assist the manual steering effort. However, when the assist torque demand from the other system is normal, but a failure is detected, the torque effective in controlling the torque steer is eliminated, and this impairs the steering feel of the vehicle operator.